,„^,^ „■ ^ ^ LAMELLIBRANCHIATA. - 571 



Whitella soofleldi.l • 



small, rounded, sitilated in the antero-dorsal angle. Posterior scar and pallial line 

 undetermined. 



In most specimens there are two or three, thin, parallel and oblique ridges on 

 each side at the extremity of the hinge. These opposed sets of ridges are separated 

 by an interval in adult examples, but it is scarcely to be questioned that in an earlier 

 stage in th^ development of the shell they represented posterior lateral teeth similar 

 to those oiCyrtodonta. This fact must have an important bearing upon the questio^n 

 of genealogy, but, in the absence of any knowledge of similar types in earlier stra,ta, 

 it is not now possible to discuss it with anything like certainty of arriving at a true 

 solution of the question. 



This is a well marked species and one that is not likely to be confounded with 

 any of the associated shells. Its nearest congeners seem to be W. scofieldi and W. 

 sterlingensis M. and W., sp., the latter particularly, but in both of these species the 

 umbonal ridge is more conspicuous and the outline diflferent. The former again has 

 a longer escutcheon and hinge, much larger anterior end and more prominent 

 postero-cardinal angle, while in the latter the shell is more oblique, the posterior 

 angle narrower, the cardinal area much wider and the beaks farther apart. 



• Formation and locality.— LoyieT Trenton limestone, near Beloit, .Wisconsin and Minneapolis, Minn. 



Whitella scofieldi Ulrich. 



PLATE XLI, FIGS. 17-21. 



t 



Whitella scofieldi TJlkich, 1890. Amer. Geol., vol. ^i, pp. 181 and 382. 



Shell of medium size, strongly convex, moderately oblique, subtrapezoidal in 

 outline, with the hinge line longer, straighter and better defined than usual. Ante- 

 rior end unusually long and wide, the outline gently rounded from the subangular 

 junction with the hinge line; basal margin slightly convex, oblique, descending to the 

 stroogly rounded postero-basal angle; posterior margin subtruncate, slightly oblique 

 and but little convex in the upper half. Umbones prominent, subcarinate behind, with 

 the beaks approximate, obliquely enrolled and situated a little more than one-third of 

 the length of the hinge line behind its anterior extremity. The umbonal ridge is a 

 conspicious feature, although becoming obsolete before reaching the postero-basal 

 margin. Posterior to the ridge the surface is distinctly concave; in front and 

 beneath it convex. Surface marked by rather distinct concentric lines of growth, 

 of which the marginal ones may, in old examples, assume a sublamellose character. 

 Escutcheon high but narrow in a dorsal view, finely striated longitudinally and not 

 extending anterior to the beaks. Internal ligamental supports appearing as a double 

 ridge in each valve beneath the posterior half of the escutcheon. Anterior hinge 



